Diesel particulate filters are used in many diesel engine systems to trap particulates generated during the combustion of hydrocarbon fuels such as conventional diesel fuel, bio-diesel, and various blends. These particulates can include a variety of organic and inorganic species, with soot and ash being of particular interest. A typical design employs a filter medium such as a ceramic or metallic medium positioned within a filter body and adapted to trap particulates carried in the exhaust stream between an inlet and an outlet of the filter.
Diesel particulate filters are often classified based on whether they “actively” or “passively” regenerate in response to soot accumulation. Regeneration generally involves inducing combustion of soot accumulated in the filter medium. In active regeneration, some external mechanism such as the introduction of a combustible fuel into the exhaust gas stream is used to raise a temperature within the filter to a point sufficient to induce combustion of the soot. In passive systems, catalysts are often used to promote combustion of accumulated soot under more ordinary filter operating temperatures. In either case, regeneration of the filter tends to do little, if anything, to remove accumulated ash. Since ash tends to accumulate much more slowly than soot, diesel particulate filters are typically cleaned of ash when the diesel particulate filter is removed from service for maintenance or other purposes.
Cleaning ash from a diesel particulate filter tends to be a relatively labor intensive process. While certain strategies have been proposed for actually monitoring accumulation of ash, they are not without shortcomings. As a result, maintenance technicians may find that filters removed from an engine for ash removal may be overloaded or under-loaded. Either case is undesirable, as an under-loaded filter may not need to have been removed, and an overloaded filter may be negatively affecting performance. The use of alternative fuels can exacerbate imperfections in conventional ash monitoring or modeling strategies. For example, bio-diesel fuels and blends tend to generate a relatively greater amount of ash then conventional diesel fuels. A viable ash sensing strategy has thus long been sought.
U.S. Pat. No. 7,157,919 to Walton discloses a method and system for detecting soot and ash concentrations in a filter. Walton proposes measuring ash build-up within a diesel particulate filter via the use of RF signals transmitted through the filter after filter regeneration. Walton indicates that after soot has been oxidized and a clean filter remains hot following regeneration, the higher temperatures can increase an “ash loss factor.” Consequently, discriminating between a hot, clean filter and a filter loaded with ash appears possible, at least in the context of Walton's technique.